Interpretive Summary: Plateau® (imazapic) herbicide is commonly used on Great Basin rangelands to control medusahead, an invasive annual grass. There is a concern that efforts to control medusahead may negatively impact desirable plant species coexisting with medusahead. This study assessed the impacts of fall applications of Plateau® on native forbs. Three of the nineteen forb species studied suffered significant density reductions one year after treatment with Plateau®, while sixteen species were unaffected. More research is needed to assess the overall benefit of using Plateau® to control medusahead where desirable, native species coexist.

Technical Abstract:
Plateau® (imazapic ammonium salt) can be used on rangelands to control non-native, invasive, and noxious weeds by inhibiting acetolactate synthase. Land managers from on the sagebrush/bunchgrass steppe region of central and southeast Oregon have concerns about the effect of Plateau® on non-targeted native forbs. We therefore designed a study to measure the mortality of selected native forbs when treated with Plateau®. In the spring of 2002, three sites were selected in Harney County, Oregon because of their rich forb component. To ensure that native plant populations were similar across treatment and control we selected 65 paired micro-plots (130 plots 1 by 1 m). Pairs had similar initial species composition, density, and vigor of a selected native forb. Between 31 October and 1 November 2002 Plateau® was applied to one micro-plot at 105 g ai h-2 (6.0 oz. product per acre) and the other was used as a control. Nineteen forb species were evaluated for change in density (number/m2) from the Plateau® treatment. Three species were reduced in number: Lomatium nudicaule (Pursh) Coult. & Rose (P = 0.0158), Zigadenus paniculatus (Nutt.) S. Wats. (P = 0.0353), and Minuartia nuttallii (Pax) Briq. (P = 0.0723). Sixteen forb species were unchanged in density. This experiment was not designed to evaluate the effectiveness of Plateau® treatments on the control of unwanted plants. Photographic analysis suggested that Plateau® treated plots had 4% less vegetative cover than control plots (P = 0.0002).